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Dermatology => Skin => Pigment
Pigment
Pigment, in biology, any chemical molecule that reflects or transmits visible light, or both. The color of a pigment depends on its selective absorption of certain wavelengths of light and its reflection of others. For example, chlorophyll, a plant pigment, absorbs light in the violet-blue and orange-red portions of the light spectrum, converting this light energy to chemical energy , and reflects light in the green and yellow portions of the spectrum. Thus, chlorophyll appears green.
Chlorophyll and many other pigments act as catalysts, substances that accelerate or facilitate chemical reactions but are not used up in the reactions. The carotenoids, a group of red, orange, and yellow pigments that occur widely in living organisms, also contain many catalytic members. Some carotenoids, such as carotene, are involved in the synthesis of vitamin A, important in vision and growth, and others act as accessory pigments in photosynthesis, transferring the light energy that they absorb to chlorophyll for conversion to chemical energy. They are synthesized by all green plants and by many fungi and bacteria and are acquired secondarily by animals through their food.
Apparently, some substances with important biological functions are only coincidentally pigments as well. Thus, the oxygen-carrying molecules in the blood of higher animals also provide the blood's coloration. Some of these pigments have taken on secondary functions. Hemoglobin, for example, is also responsible for the brilliant red coloration, important in courtship, seen in the buttocks, genitals, and faces of baboons.
Other pigments, however, are important in providing concealing coloration in animals and plants. The function of such coloration is to deceive possible predators or prey. In some systems of coloration, the concealed organism mimics its background so closely that predators cannot distinguish it. The pigmentation patterns of many tropical moths and butterflies, for example, so closely match the background patterns of the tree trunks on which they usually rest that they cannot be distinguished from the trees from only a few centimeters away. Many insects also combine pigmentation with shape to facilitate concealment. Thus, some tropical mantises blend perfectly with the orchid flowers on which they occur, both because their pigments match those of the orchids and because their bodies are generally shaped like parts of the flowers; other mantises are pigmented and shaped like leaves.
Pigments also protect organisms by providing coloration that matches that of an inedible or otherwise undesirable organism. Viceroy butterflies, for example, are pigmented orange and black in patterns that resemble those of the undesirable monarch butterfly. (Because the monarch butterfly feeds on milkweed plants and ingests alkaloids and cardiac glycosides, its ingestion has severe effects on vertebrates.)
Chemically, pigments fall into a number of large groups, but these are often arbitrarily divided into two major groups. The first group comprises pigments that contain nitrogen; it includes hemoglobins, chlorophylls, bile pigments, and dark-colored pigments called melanin, widespread in many animal groups and the chemical that is responsible for variations in the color of human skin. Related to melanins are the indigoids, of which the well known plant pigment indigo is an example. Riboflavin, which is also known as vitamin B12 , is one of a number of pale yellow to green pigments that are produced by several plant groups.
The second group is formed of pigments without nitrogen. Carotenoids are members of this group, as are the important plant pigments called flavonoids. In leaves, flavonoids selectively admit light wavelengths that are important to photosynthesis, while blocking out ultraviolet light, which is destructive to cell nuclei and proteins. Flavonoids are also important in flower color, in particular providing red and blue pigments. Bright fall colors are produced by the conversion of colorless flavonoids, called flavonols, into colored forms, called anthocyanins. Quinones provide many yellow, red, and orange pigments, including several useful dyes derived from insects that feed on plants containing the quinones. Cochineal, for example, is a red pigment obtained from the fat cells of scale insects that feed on cactus plants.
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